Fuel cell power plants are well-known and are commonly used to produce electrical energy from hydrogen containing reducing fluid fuel and oxygen containing oxidant reactant streams to power electrical apparatus such as power plants and transportation vehicles. In fuel cell power plants of the prior art, it is well known that, when an electrical circuit connected to the fuel cells is disconnected or opened and there is no longer a load across the cell, such as upon and during shut down of the cell, the presence of air on a cathode electrode along with hydrogen fuel remaining on an anode electrode, often cause unacceptable anode and cathode potentials, resulting in oxidation and corrosion of electrode catalyst and catalyst support materials and attendant cell performance degradation.
Passivation efforts have been proposed to return the cathode electrode to a passive, non-oxidative state upon shut down of the fuel cell. For example, it was thought that inert gas needed to be used to purge both an anode flow field and a cathode flow field immediately upon cell shut down to passivate the anode and cathode electrodes so as to minimize or prevent such cell performance degradation. Further, the use of an inert gas purge avoided, on start-up, the possibility of the presence of a flammable mixture of hydrogen and air, which is a safety issue. Commonly owned U.S. Pat. Nos. 5,013,617 and 5,045,414 describe using 100% nitrogen as the anode side purge gas, and a cathode side purging mixture comprising a very small percentage of oxygen (e.g. less than 1%) with a balance of nitrogen. Both of these patents also discuss the option of connecting a dummy electrical load across the cell during the start of a purging process to lower the cathode potential rapidly to between the acceptable limits of 0.3-0.7 volt. However, the costs and complexity of such stored inert gases are undesirable especially in automotive applications where compactness and low cost are critical, and where the system must be shut down and started up frequently.
Other efforts to minimize corrosion of catalyst and catalyst support materials include shutting down a fuel cell power plant by disconnecting the primary electricity using device (hereinafter, “primary load”), shutting off the air or process oxidant flow, and controlling the hydrogen fuel flow into the system and the gas flow out of the system in a manner that results in the fuel cell gases coming to equilibrium across the cells, and maintaining a gas composition of at least 0.0001% hydrogen (by volume), balance fuel cell inert gas, during shut down. This method of fuel cell shut down also includes, after disconnecting the primary load and shutting off the air supply to the cathode flow field, continuing to supply fresh fuel to the anode flow field until the remaining oxidant is completely consumed. This oxidant consumption is preferably aided by having a small auxiliary load applied across the cell, which also quickly drives down the electrode potentials. Once all the oxidant is consumed the hydrogen fuel feed is stopped. Thereafter, during continued shut down, a hydrogen concentration is monitored; and hydrogen is added, as and if necessary, to maintain the desired hydrogen concentration level.
An additional problem of fuel cell power plants that require frequent start-stop cycles, such as those used in transportation vehicles, is that, as a fuel cell power plant cools down to an ambient temperature after operation, a volume of gases within manifolds and flow fields, etc. within the plant necessarily decreases as the gases cool. Also, a lot of gaseous water within the plant condenses to a liquid phase, resulting in a further decrease in the volume of the gases within the power plant. Because the fuel cell power plant is sealed during shutdown to prohibit entry of the atmosphere, a pressure differential therefore increases between the atmosphere and the interior of the power plant as the plant cools to ambient conditions. This pressure differential causes wear on power plant valves and seals, and frequently leads to leaks of the atmosphere into fuel cell flow fields, which may in turn result in deleterious oxidation of electrode catalysts and catalysts support materials.
Known improvements to the problem of oxidation and corrosion of electrode catalysts and catalyst support materials have reduced the deleterious consequences of the presence of oxygen on the cathode electrode and a non-equilibrium of reactant fluids between the anode and cathode electrodes that result in unacceptable anode and cathode electrode potentials upon and during shut down and start up of a fuel cell. However, it has been found that even with known solutions, the presence of oxygen within an anode flow field during start up results in a reverse current leading to unacceptable, localized electrode potentials and corrosion of catalysts and catalyst support materials. Moreover, active addition of hydrogen to fuel cells of a power plant while the plant is shut down and unattended presents significant safety issues where a system failure may lead to release of potentially flammable hydrogen concentrations out of the power plant.
Consequently, there is a need for a shut down system for a fuel cell power plant that eliminates significant performance degradation of the plant, and that minimizes oxidation and corrosion within plant fuel cells at shut down of the plant, during shut down, or upon restarting the fuel cell power plant.